450 COMPARATIVE ANATOMY 



with the coelome anteriorly by means of an aperture derived from 

 the anterior nephrostomes ; only vestiges of this duct are retained 

 in the male (Fig. 339). Its formation is probably due to a division 

 of physiological labour, the primary mesonephric duct having 

 originally served to carry to the exterior the generative cells of both 

 sexes as well as the secretion of the primary urinary tubules, 

 through the nephrostomes. 



The remaining part of the mesonephric duct ( Wolffian duct) still 

 serves as a urinogenital duct in male Elasmobranchs and Amphi- 

 bians. In the Amniota, in which the mesonephros gives up its 

 function as a urinary organ, the Wolffian duct serves exclusively as 

 a spermiduct (vas deferens). 1 Its coiled anterior portion takes 

 part in the formation of the epididermis. 



The Gronads (" Generative Glands"). 



The sexual cells, which give rise to the ova and spermatozoa, 

 originate from the germinal epithelium, which corresponds to 

 a differentiation of part of the coelomic or peritoneal epithelium 

 on the dorsal side of the body-cavity on either side of the 

 mesentery, and into which the adjacent mesodermic stroma 

 penetrates ; thus a pair of gonads or " sexual glands " is formed 

 (Fig. 337). Primitively the gonads had a segmental arrangement, 

 and extended throughout a greater number of body-segments. 



The primitive germinal cells are at first all similar to one 

 another, but in the course of development a differentiation takes 

 place, resulting in the formation of a male or a female gonad, 

 i.e., a spermary (testis) or an ovary. 



The mode of development of the ova and spermatozoa is 

 briefly as follows : 



O va . The cells of the germinal epithelium grow inwards 

 amongst the connective tissue stroma of the ovary, or into its 

 cavity when hollow, in the form of clustered masses which may 

 become separated off from the periphery: certain of these cells 

 increase in size more than the others, and give rise to the ova, 

 while the smaller cells form an investment of follicle round them, 

 and may serve as nutritive material. The investing cells multiply, 

 and in Mammals a cavity containing a fluid is formed in the 

 middle of each follicle (Fig. 340) : the main mass of the follicular 



1 The mode of development of the Miillerian duct in the Amphibia, Saurop- 

 sida, and Mammalia has undergone secondary modifications, the details being 

 to some extent still under controversy ; but at any rate it appears to be certain 

 that in the Amniota its anterior end arises from a groove in the coelomic epithe- 

 lium, while its posterior part, closely connected with the mesonephric duct, is 

 formed by a gradual backward growth of an originally solid epithelial cord. 

 This later loses its connection with the coelomic epithelium, develops a lumen, 

 and breaks through into the cloaca. 



